1. Field of the Invention
The present invention relates to sealing mechanisms and more particularly to valve seal mechanisms having metal-to-metal contact for assuring positive shut-off under high temperature or fire-risk conditions.
2. Description of the Prior Art
In U.S. Pat. No. 3,642,248, issued to Robert E. Benware and owned by the assignee of the present invention, there is disclosed a valve sealing mechanism in the form of a shouldered wear ring confined in an undercut circumferential groove. The wear ring has a continuous central rib extending radially beyond the shouldered portion through the opening in the groove, the circumferential surface of the rib serving as a seating surface in mating contact with a complementary valve member sealing surface.
The wear ring in the Benware patent preferably is combined with an elastomeric O-type backing ring positioned between the bottom of the groove and the shouldered portion of the wear ring. The backing ring serves the dual function of urging the wear ring out of the groove into firm contact with the sealing surface and of sealing against leakage of fluid under pressure behind the wear ring.
An important application of butterfly valves of the general type described in U.S. Pat. No. 3,642,248 is in systems handling fluids at high pressure and temperature or flammable fluids that present a substantial risk of fire and resultant high temperatures. The patent recommends for such applications that the wear ring be fabricated of fully work hardened stainless steel and have a pair of outwardly flared side flanges extending one from each side of the shouldered portion in a generally radial direction opposite to the central rib (col. 10, line 30 through col. 11, line 32).
The flare of the side flanges is designed to provide a preloaded lateral sealing force against the sidewalls of the groove. The inner circumference of the wear ring seating surface is designed to provide a preloaded radial sealing force between the wear ring seating surface and the sealing surface of the valve member. By thus laterally and radially preloading the stainless steel wear ring, positive sealing is provided both at the seating surface-sealing surface interface and at the lines of contact between the side flanges and the walls of the groove, even if excessive temperatures should damage or destroy the elastomeric backing ring.
Although the stainless steel wear ring is an effective "fire-safe" seal, the metal-to-metal contact yields a leakage rate higher than that experienced with wear rings fabricated from various plastic materials. In addition, it is difficult and expensive to manufacture metal wear rings to the necessary dimensional tolerances for proper preloading.